Silica glass is an important material indispensable for the fabrication of semiconductor devices because of its superior heat resistance, corrosion resistance and optical characteristics, and is also used in optical fibers, photomask substrates for producing lCs, TFT substrates and so forth, showing increasingly wide uses.
The sol-gel method, which enables synthesis of high-purity silica glass at a low cost, has attracted notice. This method will be simply outlined below.
To an alkoxysilane represented by the general formula Si(OR)4 (R: an alkyl group), water (its pH may be adjusted with a base or an acid) is added to carry out hydrolysis to form a silica hydrosol (in the present invention, called a "sol"). Here, a suitable solvent is commonly added so that the alkoxysilane and the water are homogeneously mixed. This sol is gelled by leaving it to stand or raising its temperature. Thereafter, the liquid phase (an alcohol formed as a result of the hydrolysis of alkoxysilane, the solvent added, or the water) in the gel is evaporated, followed by drying to form a silica dry gel (in the present invention, called a "dry gel"). This dry gel is fired in an appropriate atmosphere to obtain silica glass.
As a problem in the process for producing silica glass by the sol-gel method described above, the gel may crack during the step of drying. As a method for preventing it from cracking, various methods are proposed in which, e.g., a partial polycondensation product of alkoxysilane is used as a material for the alkoxysilane (Japanese Patent Application Laid-open No. 61-270225), choline is used as a catalyst for the hydrolysis of alkoxysilane (Japanese Patent Application Laid-open No. 62-265128), an organic polymeric compound is added when the sol is prepared (Japanese Patent Application Laid-open No. 62-265130, the polycondensation product of a partial polycondensation product of alkoxysilane is made compositionally optimum (Japanese Patent Application Laid-open No. 3-247515), and a solvent having a higher boiling point and a smaller surface tension than water is used as a solvent (Japanese Patent Application Laid-open No. 2-14835).
However, even when these proposed methods are used, the gel may often crack during the step of drying when the dry gel is large in size, and the yield abruptly decreases with an increase in size of the dry gel.